Method for growing single crystal thin films of element semiconductor

ABSTRACT

A method for growing a single crystal thin film of an element semiconductor which comprises repeating the successive operations of feeding a single kind of gas containing the element semiconductor as a component element onto a substrate heated in a growth chamber and then exhausting the gas in the growth chamber under controlled conditions and thereby growing the single crystal thin film of said element semiconductor in a desired thickness with a precision of monomolecular layer. In an alternate method for growing a single crystal thin film of an element semiconductor, the sole gas containing the element semiconductor is continuously fed onto the substrate for a given period of time, thereby forming a single crystal thin film of element semiconductor having a desired thickness. According to the present invention, there can be obtained single crystal thin films with high quality in a high reproducibility, by simplified operating parameters and growing apparatus.

This is a division of Ser. No. 07/093,505, filed Sept. 4, 1987, now U.S.Pat. No. 4,831,831.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for growing a single crystalthin film of an element semiconductor and more specifically to a methodsuitable for growing a high-quality single crystal thin film of anelement semiconductor with the precision of monomolecular layer.

2. Description of Prior Art

As epitaxy techniques for the formation of an epitaxially grown singlecrystal layer of an element semiconductor consisting solely of oneelement such as silicon, there have been heretofore known, for example,chemical vapor deposition hereinafter referred to as "CVD"), molecularbeam epitaxy (hereinafter referred to as "MBE"), etc. Further, recently,one of the Inventors (Junichi NISHIZAWA) proposed a molecular layerepitaxy method (hereinafter abbreviated as "MLE") which made possiblethe formation of a grown single crystal layer with the precision amonomolecular layer by alternately introducing different kinds of gases.(Japanese Patent Application No. 59-153978 which has been laid open topublic inspection as Laid-Open No. 61-34928.)

However, the above-mentioned known methods have not been completelysatisfactory because of the disadvantages set forth below.

For example, in CVD, a source gas (e.g., SiH₄) is introducedsimultaneously with a carrier gas (e.g., hydrogen) into a reactionchamber and then single crystals are grown by pyrolysis. In a suchmanner, contamination by impurities is apt to occur due to theintroduction of the carrier gas, thereby causing not only deteriorationof the quality of a grown layer, but also other problems, for example,difficulties in controlling with the precision of monomolecular layer.

On the other hand, in the case of MBE in which crystal growth is carriedout under ultrahigh vacuum, it is difficult to maintain the rate ofcrystal growth constant over a long time. Further, MBE is still inferiorin the quality of the resulting crystals as compared to CVD set forthabove.

MLE has been developed with the object of overcoming the disadvantagesof these former methods, but this method is disadvantageous in that thegas feeding system is complicated, since different gases are repeatedlyalternately fed. Therefore, improvement has been awaited.

SUMMARY OF THE INVENTION

The present invention has been developed with a view of eliminating thedisadvantages associated with the above prior art. More specifically,the present invention is directed to a further improvement in the aboveMLE.

It is accordingly an object of the present invention to provide a novelmethod for growing a single crystal thin film of an elementsemiconductor with the precision of a monomolecular layer on a substrateby introducing only one kind of gas.

As described above, in the MLE, different kinds of gases are alternatelyintroduced onto a substrate so that single crystals of an elementsemiconductor are grown with the precision of the monomolecular layer.In contrast to such a prior art method, the present Inventors discovereda method for growing a single crystal thin film of an elementsemiconductor wherein the successive operations of feeding a single kindof gas containing the element semiconductor as a component element ontoa substrate heated in a growth chamber over a given period of time andexhausting the gas in the growth chamber for a given period of time arerepeated under controlled operating conditions and thereby the singlecrystal thin film of the element semiconductor is grown to a desiredthickness with the precision of a monomolecular layer.

In another feature of the present invention, a single kind of gascontaining an element semiconductor as a component element iscontinuously fed onto a substrate heated in a growth chamber undercontrolled operating conditions over a given period of time, therebygrowing a single crystal thin film of the element semiconductor in adesired thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the construction of thecrystal growing apparatus used for carrying out the present invention.

FIG. 2 is a graph showing the substrate temperature dependence of thefilm thickness grown per cycle of gas feeding and exhausting.

FIG. 3 is a graph showing the dependence of the film thickness grown percycle of gas feeding and exhausting upon the gas feeding pressure

FIGS. 4(a) to 4(d) are sequence charts showing the pulses of gasfeeding.

FIG. 5 is a graph showing the dependence of the growth rate upon the gasfeeding pressure when gas is continuously fed for a given period oftime.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As the result of the Inventors' detailed studies, the operatingconditions set forth above are practically controlled in such a mannerthat the pressure inside the growth chamber, the gas feeding time andexhausting time are in the ranges of 10⁻⁴ to 10² Pa (pascal), 1 to 60seconds and 1 to 120 seconds, respectively, and other conditions such asthe heating temperature of the substrate and the flow rate of the gasfed are appropriately adjusted.

In practicing the first feature of the present invention wherein gasfeeding and exhausting operations are alternately repeated, theoperating cycle of feeding a single gas containing an elementsemiconductor as a component element onto a substrate for apredetermined period of time and then exhausting the gas for apredetermined period of time is repeated until a single crystal thinfilm having a desired thickness is formed. The operating conditions areappropriately adjusted depending on the intended single crystal thinfilm and they are not necessarily required to be the same throughout allof the cycles.

As the gas containing an element semiconductor as a component element,SiH₂ Cl₂, SiHCl₃, SiCl₄, SiH₄ or Si₂ H₆ is preferably used in thepresent invention.

The present invention will hereinafter be described more specificallywith reference to the following examples.

EXAMPLES

FIG. 1 is a schematic diagram illustrating the construction of a crystalgrowing apparatus used for growing a single crystal thin film of anelement semiconductor in accordance to the present invention. In FIG. 1,reference numerals 1 and 2 designate a growth chamber made of a metallicmaterial, such as stainless steel, and a gate valve, respectively.Numeral 3 indicates an exhaust for evacuating the growth chamber 1 to anultrahigh vacuum and numeral 4 indicates a nozzle for introducing gas 13containing a semiconductor component element. Numeral 5 indicates aheater for heating a substrate 8 of (100) oriented silicon semiconductorcrystal. The heater 5 is made of a tungsten wire sealed within quartzand a infrared lamp 6 is disposed on the upper portion of a lamp house11 in order to heat the substrate 8. A quartz plate 10 divides the lamphouse 11 from the growth chamber 1. Leads, etc., are not shown inFIG. 1. There are also shown a pyrometer 7 for measuring temperaturesand a B-A (Bayard-Alpert) gauge 9 for measuring the degree of vacuuminside the growth chamber 1.

As a practical example of growing single crystals of IV-Group elementsemiconductor using such an apparatus, particularly silicon willhereinafter be explained.

Silicon single crystal thin films were formed using SiH₂ Cl₂(dichlorosilane) as a silicon-containing gas at various substratetemperatures while keeping the gas feeding pressure constant. FIG. 2shows the substrate temperature dependence of the thickness of siliconsingle crystal thin films grown per cycle at various substratetemperatures. Each of the thin films was grown by repeating a cycle offeeding the dichlorosiliane gas into the growth chamber 1 and evacuatingthe growth chamber 1. In each cycle, gas feeding time and exhaustingtime were 40 seconds and 20 seconds, respectively. That is, the totaltime required for one cycle was 60 seconds. The gas feeding pressure was1.3×10⁻² pascal (Pa). The thickness of the thin film grown per cyclevaries on the order of one monomolecular layer to several monolayersdepending on the temperature of the substrate, as shown in FIG. 2.Particularly, the operating conditions for growing thin films on theorder of monomolecular layer, dimolecular layer and half-molecular layerper cycle are respectively described hereinafter.

Firstly, in order to grow a monomolecular layer of Si single crystal percycle, the gate valve 2 was opened and the interior of the growthchamber 1 was evacuated to a pressure of the order of 10⁻⁷ to 10⁻⁸ Pa byusing the ultrahigh vacuum exhaust 3. The substrate 8 was heated to atemperature of 800° C. by the heater 5 or the infrared lamp 6 and itssurface was subjected to a heat cleaning for five minutes. Then, thesubstrate 8 was heated to 855° C. by the heater 5 or the infrared lamp 6and valve 12 was opened to introduce SiH₂ Cl₂ gas into the growthchamber 1 over 40 seconds while the interior pressure of the growthchamber 1 was maintained at 1.3×10⁻² Pa. Thereafter, the valve 12 wasclosed and the gas inside the growth chamber 1 was exhausted for 20seconds. Such an operating cycle grew a monomolecular layer of Si singlecrystal on the substrate 8. By repeating such a cycle under the sameconditions, monomolecular layers were successively grown one afteranother and thereby a thin film having the desired thickness could begrown with the precision of monomolecular layer For example, when theabovementioned cycle was repeated 500 times, a grown thin filmapproximately 680 Å in thickness was formed on the (100) substrate 8.

Similarly, in order to grow a dimolecular layer of silicon singlecrystal per cycle, the growth chamber 1 was evacuated to ultrahighvacuum and the surface of the substrate 8 was subjected to a heatcleaning. After heating the substrate 8 to 890° C., the valve 12 wasopened to introduce SiH₂ Cl₂ gas into the growth chamber 1 for 40seconds while maintaining the interior pressure of the growth chamber 1at 1.3×10⁻² Pa. Thereafter, the valve 12 was closed and the gas insidethe growth chamber 1 was exhausted for 20 seconds. By one cycle of suchsuccessive operations, a dimolecular layer of Si single crystal wasgrown on the substrate 8. As one practical example, when the same cyclewas repeated 500 times under the same conditions, there was obtained agrown thin film of Si single crystal with a thickness of approximately1360 Å on the (100) substrate.

Further, in order to grow a half-molecular layer of Si single crystalper cycle, the growth chamber 1 was evacuated to an ultrahigh vacuum andthe surface of the substrate 8 was subjected to a heat cleaning. Afterheating the substrate 8 to 820° C., the valve 12 was opened to introduceSiH₂ Cl₂ gas into the growth chamber 1 for 40 seconds while maintainingthe pressure inside the growth chamber 1 at 1.3×10⁻² Pa. Thereafter, thevalve 12 was closed and the gas inside the growth chamber 1 wasexhausted for 20 seconds. By one cycle of such successive operations, ahalf-molecular layer of Si single crystal was grown on the substrate 8.In an exemplary crystal growing process, when the same operations wererepeated 500 times under the same conditions, there was obtained a grownSi single crystal thin film with a thickness of approximately 340 Å onthe (100) substrate.

As set forth above, according to the present invention, it is possibleto grow single crystals of from one monolayer to several monolayers inthickness with a precision of monomolecular layer by appropriatelyvarying the temperature of the substrate under the same gas feedingpressure.

FIG. 3 is a graph showing the dependence of the film thickness grown percycle upon the gas feeding pressure in which SiH₂ Cl₂ gas was used andthe gas feeding pressure was varied while keeping the temperature of thesubstrate constant. In each cycle, the gas feeding time and theexhausting time were 40 seconds and 20 seconds. Thus, 60 seconds wasrequired for each cycle. As practical examples of growing elementsemiconductor single crystals with a precision of monomolecular layer ata substrate temperature of 850° C., we will specifically describe aboutthe operating conditions which make it possible to grow thin films ofthe order of one monolayer, dimolecular layer and half-molecular layerin thickness per cycle.

Firstly, in order to grow a monomolecular layer of Si single crystal ineach cycle, the gate valve 2 was opened and the growth chamber wasevacuated to the degree of vacuum where the interior pressure of thegrowth chamber 1 was about 10⁻⁷ to 10⁻⁸ Pa, using the ultrahigh vacuumexhaust 3. The substrate 8 was heated to 800° C. by the heater 5 or theinfrared lamp 6 and the surface of the substrate 8 was subjected to heatcleaning for about 5 minutes. Then, the substrate 8 was heated to 850°C. by the heater 5 or the infrared lamp 6 and the valve 12 was opened tointroduce SiH₂ Cl₂ gas into the growth chamber 1 for 40 seconds.Throughout the introduction of the gas, the pressure inside the growthchamber 1 was of 1.5×10⁻² Pa. Thereafter, the valve 12 was closed andthe gas in the growth chamber 1 was exhausted for 20 seconds. In onecycle of such successive operations, a monomolecular layer of Si singlecrystal was grown on the substrate 8. These successive operations wererepeated under the same conditions to grow successively monomolecularlayers. Similarly, in order to grow a dimolecular layer of Si singlecrystal per cycle, SiH₂ Cl₂ gas was fed into the growth chamber 1 for 40seconds so that the pressure inside the growth chamber 1 was brought to7.0×10⁻² Pa and then the growth chamber 1 was evacuated for 20 seconds.A dimolecular layer of Si single crystal was grown on the substrate 8per cycle.

Further, in order to grow a half-molecular layer of Si single crystalfor each cycle, SiH₂ Cl₂ gas was introduced into the growth chamber 1for 40 seconds so that the pressure within the growth chamber 1 wasbrought to 6.0×10⁻³ Pa and then the growth chamber 1 was evacuated for20 seconds. In one cycle of these successive operations, there wasobtained a grown single crystal thin film with a film thicknessequivalent to the thickness of a half-molecular layer on the substrate8.

The above examples were carried out at a substrate temperature of 850°C., but, also within the substrate temperature range of 750° to 900° C.,it is possible to grow single crystals with a precision of the order ofmonomolecular layer, as in the same manner as described in the aboveexamples, by appropriately selecting optimum conditions for gas feedingpressure and time and the evacuating time.

In the foregoing examples, the single crystal thin films having thedesired thickness were grown by repeating the successive steps of gasfeeding and evacuation under the same conditions throughout the growingprocess, as shown in a sequence chart in FIG. 4(a). However, it is notalways required to conduct every cycle under the same conditions. Ifnecessary, as shown in the sequence charts of FIGS. 4(b) to 4(d), gasfeeding pressure and time may be changed. In FIGS. 4(a) to 4(d), feedingpressure and feeding time are indicated by P₀ and P₁ and t₁ and t₃,respectively, and exhaust time is indicated by t₂. In such cases, singlecrystals are grown to a film thickness of monolayers corresponding tothe operating conditions of each cycle.

Further examples are illustrated in which element semiconductor singlecrystal thin films having a desired thickness are grown by continuouslyintroducing a single kind of gas on the substrate 8 heated in the growthchamber 1 over a given period of time.

FIG. 5 shows the dependence of growth rate of thin films upon the gasfeeding pressure. The thin films were formed using SiH₂ Cl₂(dichlorosilane) as an Si-containing gas. The temperature of thesubstrate 8 was maintained constant and the gas feeding pressure wasvaried. As noted from FIG. 5, the growth rate (μm per minute) isincreased on the order of 0.1 μm. For example, in order to form a grownthin film of 1 μm in thickness, the gate valve 2 was opened and thegrowth chamber 1 was evacuated to the degree where the pressure withinthe growth chamber 1 reached 10⁻⁶ Pa or less. The surface of thesubstrate 8 was subjected to heat cleaning and then the substrate 8 washeated to 1050° C. by the heater 5 or by the infrared lamp 6. The valve12 was opened to introduce SiH₂ Cl₂ gas into the growth chamber 1 for 10minutes, while keeping the pressure inside the growth chamber 1 at 50Pa. This procedure formed a 1 μm thick single crystal thin film onto thesubstrate 8.

As the ultrahigh vacuum exhaust in the foregoing examples,turbo-molecular pump, cryopump, ion pump or other known vacuum pumps canbe employed. Further, in the above described examples, only Si singlecrystals are described. However, the present invention can also beapplied to other IV-Group semiconductors, such as Ge. Further, thematerial of the substrate is not limited to Si. Other materials, such assapphire or spinel, can also be employed.

As described above, according to the present invention, it is possibleto grow element semiconductor single crystal thin films on a substratewith the precision of monomolecular layer using only a single kind ofgas containing a component element of an element semiconductor. Sinceonly such a single gas is used as a source gas, there can be readilyobtained single crystal thin films of high quality in a highreproducibility, by simplified operating parameters, without causingcontamination problems by impurities (e.g., oxygen, carbon, etc.), whichare associated with the use of carrier gas such as hydrogen. Further,the single crystal growth of the present invention is advantageous inthat the operating temperature for single crystal growth isapproximately 250 ° C. lower than usual epitaxial temperature(approximately 1100° C.). In addition, the present invention has anindustrial merit that the growing apparatus can be also simplified,since a gas feeding line is simplified.

What is claimed is:
 1. A method of growing a single-crystal, thin layerof silicon on a silicon substrate, from the vapor phase, at a reducedpressure, comprising the steps of:placing a silicon substrate within acrystal growth chamber; evacuating the interior of said crystal growthchamber to a reduced pressure; heating said silicon substrate to 1050°C.; and then continuously supplying only a single stream of pure SiH₂Cl₂ gas at a pressure lying within the range of gas feeding pressure(Pa) encompassed by the curve drawn through the plotting points in FIG.5 of the attached drawings, onto said silicon substrate to grown asingle-crystal, thin film of silicon on said silicon substrate, saidsingle stream of SiH₂ Cl₂ vapor being the only material that is fed intosaid crystal growth chamber during the crystal-growing process.
 2. Amethod of growing a single-crystal, thin layer of silicon on a flatsilicon substrate, from the vapor phase, under a reduced pressure,comprising the steps of:placing the flat silicon substrate in astationary, horizontal position within a crystal growth chamber in acrystal growing apparatus; evacuating said crystal growth chamber andmaintaining the pressure within said chamber at not higher than about10⁻⁶ Pa; heating said substrate to a temperature effective for cleaningsaid substrate and then heating said substrate to 1050° C.; and thencontinuously directing a single stream consisting of SiH₂ Cl₂ vapor at apressure lying within the range of gas feeding pressure (Pa) encompassedby the curve drawn through the plotting points in FIG. 5 of the attacheddrawings, against the flat upper surface of said substrate for a periodof time effective to grow a single-crystal, thin film of silicon on saidsubstrate, said single stream of SiH₂ Cl₂ vapor being the only materialthat is fed into said crystal growth chamber during the crystal growingprocess.
 3. A method of growing a single-crystal, thin layer of siliconon a silicon substrate, form the vapor phase, at a reduced pressure,comprising the steps of:placing a silicon substrate within a crystalgrowth chamber; evacuating the interior of said crystal growth chamberto a reduced pressure; heating said silicon substrate to 1050° C. andthen continuously supplying onto said silicon substrate only a singlestream of pure SiH₂ Cl₂ gas at a pressure effective to grow asingle-crystal, thin film of silicon on said silicon substrate at agrowth rate of from about 0.1 to about 0.4 μm/min., said single streamof SiH₂ Cl₂ vapor being the only material that is fed into said crystalgrowth chamber during the crystal-growing process.